1. Field of the Invention
The present invention relates to a position detection apparatus for a semiconductor wafer, a photomask, and the like using a diffraction grating and, more particularly, to a position detection apparatus suitable for alignment in accordance with a holographic alignment method.
2. Related Background Art
Recent semiconductor devices have been highly micropatterned and integrated, and alignment of a photomask (reticle) with a wafer necessary for manufacturing a semiconductor device requires a higher precision. In order to accomplish this, a holographic alignment method disclosed in, e.g., Japanese Patent Disclosure No. 192917/1983 has been proposed. In this method, two coherent light beams having the same frequency are incident from two directions. A wafer, which has a diffraction grating aligned parallel to interference fringes obtained by interference between the two light beams, is arranged along a common optical path of the two light beams. Then, light beams reflected, transmitted, or diffracted by the diffraction grating are again interfared with each other, and a position of the interference fringes relative to the position of the diffraction grating is detected based on a signal obtained by photoelectrically converting the interference intensity. In other words, this method detects a change in interference intensity due to changes in phases of first diffracted light generated in a specific direction upon radiation of one of the two light beams onto the diffraction grating and second diffracted light generated in another specific direction upon radiation of the other light beam onto the diffraction grating according to movement of the diffraction grating relative to the two light beams. This method can improve a detection precision, but creates a serious problem in practical applications. More specifically, changes in intensities of first and second diffracted light beams due to a change in intensity of a light source for generating two coherent light beams, a change in intensity ratio between two light beams, and mismatching of a diffraction grating directly cause position detection errors. Although the change in intensity of the light source and the change in intensity ratio between two light beams can be considerably eliminated during apparatus design and the manufacture thereof, time variations cannot be eliminated. Since the changes in intensities of the first and second diffracted light beams occur depending on a shape of the diffraction grating and its surface condition, it is difficult to compensate for them at the apparatus side.
Furthermore, a photo-electrical signal, whose level varies in accordance with deviations, cannot be obtained unless the interference fringes formed by the two light beams are moved relative to the diffraction grating on the wafer. This means that precise alignment is disabled until the interference fringes and the diffraction grating are relatively moved by one period. This also means that alignment time cannot be shortened in the apparatus.